The present invention relates, in general, to semiconductor devices, and more particularly to semiconductor devices having heat spreading layers.
Generally, electronic circuits on semiconductor die generate heat during operation. This is especially true with power semiconductor devices. To alleviate this problem it is common practice to affix the semiconductor die to a heat sink. Conventionally, a semiconductor die is affixed to a heat conductive metal acting as a heat sink which dissipates the heat. One predominant problem that occurs with the conventional method is that the semiconductor die cannot be connected directly to the heat conductive metal. Typically, the semiconductor die has a much different thermal coefficient of expansion (TCE) than the heat conductive metal. Thus, if the semiconductor die is connected directly to the heat conductive metal, the die would be physically damaged by the stresses resulting from differing expansions during temperature cycling.
Semiconductor die, and especially power die, are conventionally bonded to a metal substrate of aluminum by interposing a dielectric layer and possibly other layers of various materials between the aluminum substrate and the die. The other layers typically are selected to permit the die to be soldered to the layered stack of materials so as to maximize heat transfer between the die and the substrate without introducing materials that are incompatible with the TCE of the die. The other layers are deposited on the aluminum substrate in a desired pattern. The die is then soldered to the layered stack using a solder preform.
In a typical prior art structure where the metal substrate is aluminum and the dielectric consists of a layer of alumina (Al.sub.2 O.sub.3) on the substrate, a metal layer of nickel or copper is interposed between the silicon die and the alumina to provide a surface to which the die is readily soldered. However, the TCE's of nickel and copper are so different from that of silicon that the thermal stresses are unacceptable in most applications. Other metals, such as molybdenum and tungsten have an acceptable TCE's and exhibit good thermal and electrical conductivity, however, a solderable layer is necessary for good adhesion between molybdenum or tungsten and silicon.
Currently, a backmetal layer is deposited on the back surface of the silicon die and a piece part of molybdenum is soldered to the backmetal layer. To reduce the number of piece parts in stock, typically a single size piece part of molybdenum is used for all sizes of silicon die. In order to accommodate all sizes of die, a common piece part of molybdenum must be large enough to receive the largest size die. This results in too large a piece part for small die, which reduces packaging efficiency because an unnecessarily large package footprint is required to support the large molybdenum piece part. Also, this creates a waste of materials. In addition, soldering the silicon die to the piece part of molybdenum produces excessive heat that can damage and/or change the characteristics of electronic circuits on the die. Furthermore, the solder adds thermal resistance and inhibits heat dissipation due to poor thermal conductivity and the presence of voids between the silicon die and the piece part molybdenum.
What is needed is a heat spreading layer for attaching a semiconductor die to a metal substrate that provides enhanced heat conduction, that requires fewer processing steps than conventional processes, that has TCE compatibility with the semiconductor die, and that does not require solder to attach it to the semiconductor die.